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Well,<br />
The shortest is to read <a href="https://blogs.technet.microsoft.com/exchange/2015/10/12/the-exchange-2016-preferred-architecture/" rel="ugc">recommended approach</a>, highly recommended.<br />
Plan <a href="https://technet.microsoft.com/en-us/library/dd638104(v=exchg.160).aspx" rel="ugc">appropriately</a>.
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Yes you would need the ADC if you want to have DR setup
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Requested information provided
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I currently have a 2 server DAG internally in the office. &nbsp;This is healthy and working. &nbsp;I want to host a 3rd member offsite in another state so in the event that the office power is out, a disaster occurs, and so on, mail will continue to flow, users can access email via smart device, laptop, etc. &nbsp;I'm sure this is a pretty general question, but how do I proceed??!! &nbsp;I have access to a colo and I can setup a VPN between the 2 sites if need be. &nbsp;Will I need a DC at the offsite location as well as Exchange? &nbsp;I&quot;m sure mail flow can be managed with MX records and I've come across some 3rd party apps that can act as an https redirector for OWA.
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I currently have a 2 server DAG internally in the office.  This is healthy and working.  I want to host a 3rd member offsite in another state so in the event that the office power is out, a disaster occurs, and so on, mail will continue to flow, users can access email via smart device, laptop, etc.  I'm sure this is a pretty general question, but how do I proceed??!!  I have access to a colo and I can setup a VPN between the 2 sites if need be.  Will I need a DC at the offsite location as well as Exchange?  I"m sure mail flow can be managed with MX records and I've come across some 3rd party apps that can act as an https redirector for OWA.

Exchange 2010 offsite DAG

Database Availability Group (DAG)

Exchange is the server side of a collaborative application product that is part of the Microsoft Server infrastructure. Exchange's major features include email, calendaring, contacts and tasks, support for mobile and web-based access to information, and support for data storage.